We thank Chris Benner for implementing the X. tropicalis genome (xenTro2, xenTro2r) into HOMER and the Gilchrist lab for discussions on post-sequencing analysis. I.P. assisted the GO term analysis. G.E.G and J.C.S. were supported by the Wellcome Trust and are now supported by the Medical Research Council (program number U117597140).

<ol>
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E., Guptaa, R., Chuonga, E., Baker, J. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chromatin+immunoprecipitation+and+deep+sequencing+in+Xenopus+tropicalis+and+Xenopus+laevis.">Chromatin immunoprecipitation and deep sequencing in Xenopus tropicalis and Xenopus laevis.</a> Methods. 66 (3), 410-421 (2014).</li><li>Metz, B., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Identification+of+formaldehyde-induced+modifications+in+proteins:+reactions+with+model+peptides.">Identification of formaldehyde-induced modifications in proteins: reactions with model peptides.</a> J Biol Chem. 279 (8), 6235-6243 (2004).</li><li>Landt, S. 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The authors declare that they have no competing financial interests.

Our protocol outlines how to make and analyze genome-wide chromatin profiles from Xenopus embryos. It covers every step from cross-linking proteins to endogenous loci in vivo to processing millions of reads representing enriched genomic sites in silico. Since increasing numbers of genome drafts are available, this protocol should be applicable to other model and non-model organisms. The most important experimental section, which sets this protocol apart from previous work8,31,33,34, is the post-fixation procedure to extract cross-linked nuclei. It facilitates efficient chromatin solubilisation and shearing and easy upscaling. Together with improved efficiencies of library preparation this protocol allows the construction of high-complexity ChIP-Seq libraries from half to two million cells expressing the chromatin-associated epitope of interest. For ChIP-qPCR experiments, a few ten thousand of these cells are normally enough to check for DNA enrichment at perhaps six distinct genomic loci. These numbers are conservative estimates, but may vary depending on protein expression level, antibody quality, cross-linking efficiency, and epitope accessibility. As a guide, a single Xenopus embryo contains about 4,000 cells at the mid-blastula stage (8.5 after Nieuwkoop and Faber29), 40,000 cells at the late gastrula stage (12) and 100,000 cells at the early tailbud stage (20).
The exact fixation time for efficient immunoprecipitation needs to be determined empirically by ChIP-qPCR (section 10). In general, longer fixation times are required if the experiment involves X. laevis embryos, early developmental stages, and weak (or indirect) DNA binding properties. However, it is not recommended fixing Xenopus embryos longer than 40 min, or processing more embryos than indicated (section 3), as chromatin shearing becomes less efficient. It is important not to use any glycine after fixation as this common step for quenching formaldehyde can make nuclear extraction from yolk-rich embryos very difficult. Currently, the reason for this is not known. It is conceivable that the formaldehyde-glycine adduct further reacts with N-terminal amino-groups or arginine residues35.
The antibody is key to any ChIP experiment and sufficient controls need to be carried out to show its specificity for the epitope of interest (see guidelines by Landt et al.36). If no ChIP-grade antibody is available, the introduction of corresponding epitope-tagged fusion proteins may be a legitimate alternative as these proteins can occupy endogeneous binding sites37. In this case, uninjected embryos are best to use as a negative control rather than a ChIP with non-specific serum. This strategy may also be applied if the protein of interest is expressed at low levels resulting in the poor recovery of enriched DNA.
As for making ChIP-Seq libraries, because of the low amount of DNA in use, it is recommended to opt for procedures that reduce the number of cleaning steps and to combine reactions to keep any loss of DNA at a minimum. The adaptors and primers need to be compatible with multiplex sequencing and the NGS platform (see Table of Specific Materials/Equipment). If using Y-adaptors (containing long single-stranded arms), it is critical to pre-amplify the library with three to five rounds of PCR before size-selecting DNA inserts (e.g., 100 to 300 bp) by gel electrophoresis. Single-stranded ends cause DNA fragments to migrate heterogeneously. Trial runs with various amounts of input DNA (e.g., 0.1, 0.5, 1, 2, 5, 10 and 20 ng) are recommended to determine the total number of PCR cycles (less than or equal to 18 cycles) required to make a size-selected library of 100 to 200 ng. Reducing the number of PCR cycles renders the sequencing of redundant reads less likely. Solid phase reversible immobilization beads are good cleaning up reagents to efficiently recover the DNA of interest and reliably remove any free adaptors and dimers from ligation and PCR reactions.
In terms of number, type and length of reads, around 20 to 30 million single-end reads of 36 bp is enough for most ChIP-Seq experiments to cover the whole Xenopus genome with sufficient depth. The most prevalent NGS machines are routinely capable of meeting these criteria. However, it may be beneficial to increase the number of reads if a broad distributions of reads is expected, as observed with histone modifications, rather than sharp peaks. For many ChIP-Seq experiments, 4 to 5 differently indexed libraries can be pooled and sequenced in one flow cell lane using a high-performance NGS machine. Sometimes is also advisable to extend the read length and sequence both ends of the DNA template (paired-end) to increase mappability when analyzing chromatin within repetitive genomic regions.
This protocol has been applied successfully to a wide variety of chromatin features such as transcription factors, signaling mediators and post-translational histone modifications. However, embryos acquire an increasing degree of cellular heterogeneity as they develop and chromatin profiles become harder to interpret. Promising steps have been made in Arabidopsis and Drosophila to tissue-specifically profile chromatin landscapes by extracting cell type-specific nuclei38,39. Our protocol includes a nuclear extraction step, which could pave the way for tissue-specific ChIP-Seq in other embryos.

The first attempts to characterize protein-DNA interactions in vivo were reported about 30 years ago in an effort to understand RNA polymerase-mediated gene transcription in bacteria and in the fruit fly1,2. Since then, the use of immunoprecipitation to enrich distinct chromatin features (ChIP) has been widely adopted to capture binding events and chromatin states with high efficiency3. Subsequently, with the emergence of powerful microarray technologies, this method led to the characterization of genome-wide chromatin landscapes4. More recently, chromatin profiling has become even more comprehensive and high-resolution, because millions of co-immunoprecipitated DNA templates can now be sequenced in parallel and mapped to the genome (ChIP-Seq)5. As increasing numbers of genome assemblies are available, ChIP-Seq is an attractive approach to learn more about the genome regulation that underlies biological processes. 
Here we provide a protocol to perform ChIP-Seq on yolk-rich embryos such as those of the frog Xenopus. Drafts of the genomes of both widely used Xenopus species&#8212;X. tropicalis and X. laevis&#8212;have now been released by the International Xenopus Genome Consortium6. The embryos of Xenopus species share many desirable features that facilitate and allow the interpretation of genome-wide chromatin studies, including the production of large numbers of high-quality embryos, the large size of the embryos themselves, and their external development. In addition, the embryos are amenable to classic and novel manipulations like cell lineage tracing, whole-mount in situ hybridisation, RNA overexpression, and TALEN/CRISPR-mediated knockout technology.
The following protocol builds on the work of Lee et al., Blythe et al. and Gentsch et al.7-9. Briefly, Xenopus embryos are formaldehyde-fixed at the developmental stage of interest to covalently bind (cross-link) proteins to their associated genomic DNA. After nuclear extraction, cross-linked chromatin is fragmented to focus subsequent sequencing on specific genomic binding or modification sites, and to minimize the contributions of flanking DNA sequences. Subsequently, the chromatin fragments are immunoprecipitated with a ChIP-grade antibody to enrich those containing the protein of interest. The co-immunoprecipitated DNA is stripped from the protein and purified before creating an indexed (paired-end) library for next-generation sequencing (NGS). At the end, simple command lines are offered for the post-sequencing analysis of ChIP-Seq data.

<table border="0" cellpadding="0" cellspacing="0">
	<tbody>
		<tr>
			<td>1/16 inch tapered microtip</td>
			<td>Qsonica</td>
			<td>4417</td>
			<td>This microtip is compatible with Sonicator 3000 from Misonix and Q500/700 from Qsonica.</td>
		</tr>
		<tr>
			<td>8 ml glass sample vial with cap</td>
			<td>Wheaton</td>
			<td>224884</td>
			<td>8 ml clear glass sample vials for aqueous samples with 15-425 size phenolic rubber-lined screw caps.</td>
		</tr>
		<tr>
			<td>Adaptor</td>
			<td>IDT or Sigma</td>
			<td>NA</td>
			<td>TruSeq universal adaptor, 
AATGATACGGCGACCACCGAG 
ATCTACACTCTTTCCCTACAC 
GACGCTCTTCCGATC*T. TruSeq indexed adaptor, 
P-GATCGGAAGAGCACACGTC 
TGAACTCCAGTCAC &#8208;NNNNNN&#8208; 
ATCTCGTATGCCGTCT TCTGCTT*G. *, phosphorothioate bondphosphate group at 5&#39; end. NNNNNN, index (see TruSeq ChIP Sample Preparation Guide for DNA sequence). Order adaptors HPLC purified. Adaptors can be prepared by combining equimolar amounts (each 100 &#181;M) of the universal and the indexed adaptor and cooling them down slowly from 95 &#176;C to room temperature. Use 1.5 pmol per ng of input DNA. Store at -20 &#176;C.</td>
		</tr>
		<tr>
			<td>b2g4pipe (software)</td>
			<td>Blast2GO</td>
			<td>non-commercial</td>
			<td>http://www.blast2go.com/data/blast2go/b2g4pipe_v2.5.zip</td>
		</tr>
		<tr>
			<td>BLAST+ (software)</td>
			<td>Camacho et al.</td>
			<td>non-commercial</td>
			<td>http://blast.ncbi.nlm.nih.gov/Blast.cgi?PAGE_TYPE=BlastDocs&#38;DOC_TYPE=Download</td>
		</tr>
		<tr>
			<td>Bowtie (software)</td>
			<td>Langmead et al.</td>
			<td>non-commercial</td>
			<td>http://bowtie-bio.sourceforge.net/index.shtml</td>
		</tr>
		<tr>
			<td>cisFinder (software)</td>
			<td>Sharov et al.</td>
			<td>non-commercial</td>
			<td>http://lgsun.grc.nia.nih.gov/CisFinder/</td>
		</tr>
		<tr>
			<td>Chip for capillary electrophoresis</td>
			<td>Agilent Technologies</td>
			<td>5067-1504</td>
			<td>Load this chip with 1 &#181;l DNA for library quality control. Store at 4 &#176;C.</td>
		</tr>
		<tr>
			<td>Chip-based capillary electrophoresis system</td>
			<td>Agilent Technologies</td>
			<td>G2940CA</td>
			<td>The Agilent 2100 BioAnalyzer is used to check the quality of ChIP-Seq libraries. Keep reagents at 4 &#176;C.</td>
		</tr>
		<tr>
			<td>ChIP-Seq library preparation kit (KAPA Hyper Prep Kit)</td>
			<td>Kapa Biosystems</td>
			<td>KK8504</td>
			<td>Kit contains KAPA end repair and A-tailing enzyme mix, end Repair and A-tailing buffer, DNA ligase, ligation buffer, KAPA HiFi HotStart ReadyMix (2X), and KAPA library amplification primer mix (10X) (see also PCR primers). Adaptors are not included. Store at -20 &#176;C.</td>
		</tr>
		<tr>
			<td>ChIP-Seq library preparation kit (alternative, ThruPLEX-FD Prep Kit)</td>
			<td>Rubicon Genomics</td>
			<td>R40048</td>
			<td>Kit uses their own stem-loop adaptors and primers. This kit eliminates intermediate purification steps and is as sensitive as the KAPA Hyper Prep Kit. Store at -20 &#176;C.</td>
		</tr>
		<tr>
			<td>Cluster3 (software)</td>
			<td>de Hoon et al.</td>
			<td>non-commercial</td>
			<td>http://bonsai.hgc.jp/~mdehoon/software/cluster</td>
		</tr>
		<tr>
			<td>FastQC (software)</td>
			<td>Simon Andrews</td>
			<td>non-commercial</td>
			<td>http://www.bioinformatics.babraham.ac.uk/projects/fastqc</td>
		</tr>
		<tr>
			<td>Fluorometer</td>
			<td>life technologies</td>
			<td>Q32866</td>
			<td>Qubit 2.0 Fluorometer</td>
		</tr>
		<tr>
			<td>Fluorometer reagents</td>
			<td>life technologies</td>
			<td>Q32851</td>
			<td>The kit provides concentrated assay reagent, dilution buffer, and pre-diluted DNA standards for the Qubit fluorometer. Store DNA standards at 4 &#176;C, buffer and dye at room temperature.</td>
		</tr>
		<tr>
			<td>Formaldehyde</td>
			<td>Sigma</td>
			<td>F8775-4X25ML</td>
			<td>Formaldehyde solution, for molecular biology, 36.5-38% in H2O, stabilised with 10-15% methanol. Store at room temperature. CAUTION: Formaldehyde is corrosive and highly toxic.</td>
		</tr>
		<tr>
			<td>Gel (E-Gel EX agarose , 2%)</td>
			<td>life technologies</td>
			<td>G4010</td>
			<td>Pre-cast gel with 11 wells, openable format. Leave one lane between ladder and library empty to avoid cross-contamination. Store gels at room temperature.</td>
		</tr>
		<tr>
			<td>Gel electrophoresis system</td>
			<td>life technologies</td>
			<td>G6465</td>
			<td>E-Gel iBase and E-Gel Safe Imager combo kit for size-selecting ChIP-Seq libraries.</td>
		</tr>
		<tr>
			<td>Gel extraction kit</td>
			<td>Qiagen</td>
			<td>28706</td>
			<td>Store all reagents at room temperature. Use 500 &#181;l of QG buffer per 100 mg of 2% agarose gel slice to extract DNA. Use MinElute columns (from MinElute PCR purification kit) to elute DNA twice.</td>
		</tr>
		<tr>
			<td>HOMER (software)</td>
			<td>Chris Benner</td>
			<td>non-commercial</td>
			<td>http://homer.salk.edu/homer/index.html</td>
		</tr>
		<tr>
			<td>Hybridization oven</td>
			<td>Techne</td>
			<td>FHB1D</td>
			<td>Hybridizer HB-1D</td>
		</tr>
		<tr>
			<td>IGV (software)</td>
			<td>Robinson et al.</td>
			<td>non-commercial</td>
			<td>http://www.broadinstitute.org/igv/home</td>
		</tr>
		<tr>
			<td>Illumina CASAVA-1.8 quality filter (software)</td>
			<td>Assaf Gordon</td>
			<td>non-commercial</td>
			<td>http://cancan.cshl.edu/labmembers/gordon/fastq_illumina_filter</td>
		</tr>
		<tr>
			<td>Java TreeView (software)</td>
			<td>Alok Saldanha</td>
			<td>non-commercial</td>
			<td>http://jtreeview.sourceforge.net</td>
		</tr>
		<tr>
			<td>Laboratory jack</td>
			<td>Edu-Lab</td>
			<td>CH0642</td>
			<td>This jack is used to elevate sample in sound enclosure for sonication.</td>
		</tr>
		<tr>
			<td>Ladder, 100 bp</td>
			<td>New England BioLabs</td>
			<td>N3231</td>
			<td>Keep 1x solution at room temperature. Store stock at -20 &#176;C.</td>
		</tr>
		<tr>
			<td>Ladder, 1 kb</td>
			<td>New England BioLabs</td>
			<td>N3232</td>
			<td>Keep 1x solution at room temperature. Store stock at -20 &#176;C.</td>
		</tr>
		<tr>
			<td>Low-retention 1.5-ml microcentrifuge tubes</td>
			<td>life technologies</td>
			<td>AM12450</td>
			<td>nonstick, RNase-free microfuge tubes, 1.5 ml</td>
		</tr>
		<tr>
			<td>MACS2 (software)</td>
			<td>Tao Liu</td>
			<td>non-commercial</td>
			<td>https://github.com/taoliu/MACS</td>
		</tr>
		<tr>
			<td>Magnetic beads</td>
			<td>life technologies</td>
			<td>11201D</td>
			<td>These Dynabeads are superparamagnetic beads with affinity purified polyclonal sheep anti-mouse IgG covalently bound to the bead surface. Store at 4 &#176;C.</td>
		</tr>
		<tr>
			<td>Magnetic beads</td>
			<td>life technologies</td>
			<td>11203D</td>
			<td>These Dynabeads are superparamagnetic beads with affinity purified polyclonal sheep anti-rabbit IgG covalently bound to the bead surface. Store at 4 &#176;C.</td>
		</tr>
		<tr>
			<td>Magnetic beads</td>
			<td>life technologies</td>
			<td>10001D</td>
			<td>These Dynabeads are superparamagnetic beads with recombinant protein A covalently bound to the bead surface. Store at 4 &#176;C.</td>
		</tr>
		<tr>
			<td>Magnetic beads</td>
			<td>life technologies</td>
			<td>10003D</td>
			<td>These Dynabeads are superparamagnetic beads with recombinant protein G covalently bound to the bead surface. Store at 4 &#176;C.</td>
		</tr>
		<tr>
			<td>Magnetic rack</td>
			<td>life technologies</td>
			<td>12321D</td>
			<td>DynaMag-2 magnet</td>
		</tr>
		<tr>
			<td>MEME</td>
			<td>Bailey et al.</td>
			<td>non-commercial</td>
			<td>http://meme.nbcr.net/meme/</td>
		</tr>
		<tr>
			<td>Na3VO4</td>
			<td>New England BioLabs</td>
			<td>P0758</td>
			<td>Sodium orthovanadate (100 mM) is a commonly used general inhibitor for protein phosphotyrosyl phosphatases. Store at -20 &#176;C.</td>
		</tr>
		<tr>
			<td>NaF</td>
			<td>New England BioLabs</td>
			<td>P0759</td>
			<td>Sodium fluoride (500 mM) is commonly used as general inhibitor of phosphoseryl and phosphothreonyl phosphatases. Store at -20 &#176;C.</td>
		</tr>
		<tr>
			<td>NGS machine</td>
			<td>Illumina</td>
			<td>SY-301-1301</td>
			<td>Genome Analyzer IIx</td>
		</tr>
		<tr>
			<td>NGS machine (high performance)</td>
			<td>Illumina</td>
			<td>SY-401-2501</td>
			<td>HiSeq</td>
		</tr>
		<tr>
			<td>Normal serum (antibody control)</td>
			<td>Santa Cruz Biotechnology</td>
			<td>sc-2028</td>
			<td>Use as control for goat polyclonal IgG antibodies in ChIP-qPCR experiments. Store at 4 &#176;C.</td>
		</tr>
		<tr>
			<td>Normal serum (antibody control)</td>
			<td>Santa Cruz Biotechnology</td>
			<td>sc-2025</td>
			<td>Use as control for mouse polyclonal IgG antibodies in ChIP-qPCR experiments. Store at 4 &#176;C.</td>
		</tr>
		<tr>
			<td>Normal serum (antibody control)</td>
			<td>Santa Cruz Biotechnology</td>
			<td>sc-2027</td>
			<td>Use as control for rabbit polyclonal IgG antibodies in ChIP-qPCR experiments. Store at 4 &#176;C.</td>
		</tr>
		<tr>
			<td>Nucleic acid staining solution</td>
			<td>iNtRON</td>
			<td>21141</td>
			<td>Use RedSafe nucleic acid staining solution at 1:50,000. Store at room temperature.</td>
		</tr>
		<tr>
			<td>Orange G</td>
			<td>Sigma</td>
			<td>O3756-25G</td>
			<td>1-Phenylazo-2-naphthol-6,8-disulfonic acid disodium salt. Store at 4 &#176;C.</td>
		</tr>
		<tr>
			<td>PCR primers</td>
			<td>e.g., IDT or Sigma</td>
			<td>NA</td>
			<td>Primers to enrich adaptor-ligated DNA fragments by PCR: AATGATACGGCGACCACCGA*G and CAAGCAGAAGACGGCATACGA*G, phosphorothioate bond. Primers designed by Ethan Ford. Combine primers at 5 &#181;M each. Use 5 &#181;l in a 50 &#181;l PCR reaction. Store at -20 &#176;C.</td>
		</tr>
		<tr>
			<td>MinElute PCR purification kit&#160;</td>
			<td>Qiagen</td>
			<td>28006</td>
			<td>for purification of ChIP-qPCR and shearing test samples. Store MinElute spin columns at 4 &#176;C, all other buffers and collection tubes at room temperature.</td>
		</tr>
		<tr>
			<td>Phenol:chloroform:isoamyl alcohol (25:24:1, pH 7.9)</td>
			<td>life technologies</td>
			<td>AM9730</td>
			<td>Phenol:Chloroform:IAA (25:24:1) is premixed and supplied at pH 6.6. Use provided Tris alkaline buffer to raise pH to 7.9. Store at 4 &#176;C. CAUTION: phenol:chloroform:isoamyl alcohol is corrosive, highly toxic and combustible.</td>
		</tr>
		<tr>
			<td>Primer3 (software)</td>
			<td>Steve Rozen &#38; Helen Skaletsky</td>
			<td>non-commercial</td>
			<td>http://biotools.umassmed.edu/bioapps/primer3_www.cgi</td>
		</tr>
		<tr>
			<td>Protease inhibitor tablets</td>
			<td>Roche</td>
			<td>11836170001</td>
			<td>cOmplete, Mini, EDTA-free. Use 1 tablet per 10 ml. Store at 4 &#176;C.</td>
		</tr>
		<tr>
			<td>Protease inhibitor tablets</td>
			<td>Roche</td>
			<td>11873580001</td>
			<td>cOmplete, EDTA-free. Use 1 tablet per 50 ml. Store at 4 &#176;C.</td>
		</tr>
		<tr>
			<td>Proteinase K</td>
			<td>life technologies</td>
			<td>AM2548</td>
			<td>proteinase K solution (20 &#181;g/&#181;l). Store at -20 &#176;C.</td>
		</tr>
		<tr>
			<td>RNase A</td>
			<td>life technologies</td>
			<td>12091-039</td>
			<td>RNase A (20 &#181;g/&#181;l). Store at room temperature.</td>
		</tr>
		<tr>
			<td>Rotator</td>
			<td>Stuart</td>
			<td>SB3</td>
			<td>Rotator SB3</td>
		</tr>
		<tr>
			<td>SAMtools (software)</td>
			<td>Li et al.</td>
			<td>non-commercial</td>
			<td>http://samtools.sourceforge.neta</td>
		</tr>
		<tr>
			<td>Solid phase reversible immobilisation beads</td>
			<td>Beckman Coulter</td>
			<td>A63882</td>
			<td>The Agencourt AMPure XP beads are used to minimise adaptor dimer contamination in ChIP-Seq libraries. Store at 4 &#176;C.</td>
		</tr>
		<tr>
			<td>Sonicator 3000</td>
			<td>Misonix/Qsonica</td>
			<td>NA</td>
			<td>Newer models are now available. Q125, Q500 or Q700 are all suitable for shearing crosslinked chromatin.</td>
		</tr>
		<tr>
			<td>Sound enclosure</td>
			<td>Misonix/Qsonica</td>
			<td>NA</td>
			<td>optional: follow the manufacturer&#39;s recommendation to obtain the correct sound enclosure.</td>
		</tr>
		<tr>
			<td>Thermomixer</td>
			<td>eppendorf</td>
			<td>22670000</td>
			<td>Thermomixer for 24 x 1.5 mL tubes. Precise temperature control from 4 &#176;C above room temperature to 99 &#176;C.</td>
		</tr>
	</tbody>
</table>

genome-wide snapshot of chromatin regulators and states in xenopus embryos by chip-seq

The recruitment of chromatin regulators and the assignment of chromatin states to specific genomic loci are pivotal to cell fate decisions and tissue and organ formation during development. Determining the locations and levels of such chromatin features in vivo will provide valuable information about the spatio-temporal regulation of genomic elements, and will support aspirations to mimic embryonic tissue development in vitro. The most commonly used method for genome-wide and high-resolution profiling is chromatin immunoprecipitation followed by next-generation sequencing (ChIP-Seq). This protocol outlines how yolk-rich embryos such as those of the frog Xenopus can be processed for ChIP-Seq experiments, and it offers simple command lines for post-sequencing analysis. Because of the high efficiency with which the protocol extracts nuclei from formaldehyde-fixed tissue, the method allows easy upscaling to obtain enough ChIP material for genome-wide profiling. Our protocol has been used successfully to map various DNA-binding proteins such as transcription factors, signaling mediators, components of the transcription machinery, chromatin modifiers and post-translational histone modifications, and for this to be done at various stages of embryogenesis. Lastly, this protocol should be widely applicable to other model and non-model organisms as more and more genome assemblies become available.

Equivalent results to those presented here are expected if the protocol is well executed and the antibody in use is of ChIP-grade quality (see discussion). This protocol allows the extraction of nuclei from formaldehyde-fixed Xenopus embryos and the efficient shearing of chromatin by sonication (Figure 1A-C). Sheared chromatin shows an asymmetric distribution of DNA fragments mainly ranging from 100 to 1,000 bp and peaking between 300 and 500 bp (Figure 1C). A minimal 50 pg of immunoprecipitated DNA is required to successfully make an indexed paired-end ChIP-Seq library with similarly sized DNA inserts (Figure 2A). The library should be largely devoid of adaptor dimers, which can be seen on the electropherogram at approximately 120 bp.
Upon sequencing-by-synthesis, pre-processed reads are mapped to the genome (Figure 2B, C). In a successful experiment with X. tropicalis embryos, normally 50 to 70% of single-end reads of 40 bp can be mapped uniquely to the genome assembly of v7.1 with maximally two mismatches. While input reads align quite uniformly across the genome, the alignment of ChIP reads results in strand-specific enrichments that flank the chromatin feature of interest. This is because all fragments are sequenced from the 5&#8217; end (Figure 2C)25. Extending the alignment in reading direction to an average fragment size produces accurate profiles for single chromatin features such as transcription factor binding events. These DNA occupancies appear as peaks when visualized in IGV or any other compatible genome browser. Peak callers like MACS are used to determine the location of these peaks (Figure 3A). This way tens of thousands of binding sites have been determined in the X. tropicalis genome for T-box transcription factors such as VegT26. ChIP-qPCR experiments should confirm the local enrichment found by ChIP-Seq (Figure 3B).
ChIP-Seq experiments allow exploring genome-wide characteristics of chromatin features. For example, calculating the read distribution over genomic elements such as transcription start and termination sites may highlight any spatial binding preferences around genes (Figure 3C). Similarly, a heatmap of read distributions at peak locations is used to compare different chromatin features at a genome-wide scale (Figure 3D). Certain transcription factors bind DNA sequence-specifically. De novo motif analysis of genomic DNA underlying peaks can retrieve this kind of information including co-enriched motifs of potential co-factors (Figure 3E). The great majority of target genes show DNA occupancy at a lower rather than higher level (Figure 3F). This scale-free feature seems to be quite common among transcription factors and suggests that only a small fraction of target genes are directly regulated with biological relevance27,28. The analysis of enriched GO terms or other attributes such as the differential expression of target genes may further reveal insights into the biological function of the chromatin feature in the Xenopus embryo (Figure 3G).
<img alt="Figure 1" src="/files/ftp_upload/52535/52535fig1highres.jpg" /> 
Figure 1.&#160;Chromatin immunoprecipitation procedure for Xenopus embryos. (A) Embryos are formaldehyde-fixed at the developmental stage of interest to covalently bind (cross-link) any proteins associated with genomic DNA. Upon nuclear extraction (B), cross-linked chromatin is fragmented to narrow down genomic DNA binding or chromatin modification sites by minimizing the flanking DNA sequence (C). Subsequently, the chromatin fragments are immunoprecipitated with a ChIP-grade antibody to enrich those containing the epitope of interest (D). The co-immunoprecipitated DNA is stripped off the protein and purified (E) before creating the ChIP fragment library for NGS (Figure 2). <a href="https://www.jove.com/files/ftp_upload/52535/52535fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/52535/52535fig2highres.jpg" /> 
Figure 2.&#160;ChIP-Seq library preparation, sequencing-by-synthesis, mapping and peak calling. (A) The electropherogram displays a good ChIP-Seq library with DNA templates of 250 to 450 bp. These templates entail the DNA insert of interest flanked by the universal (58 bp) and the indexed (63 bp) adaptor. (B) Millions of clusters, with each cluster containing identical templates, are sequenced base by base in the presence of all four nucleotides possessing reversible, distinct fluorophore and identical termination properties. Fluorescent images are processed in real time to call corresponding bases, which ultimately are assembled into reads. (C) Only reads that map uniquely to the Xenopus genome are kept. As all fragments are sequenced from the 5&#8217; end, the mapping of ChIP reads results in strand-specific peaks that flank the chromatin feature of interest. Thereby, peak callers detect the enrichment that originates from immunoprecipitation and extend the reads to an average fragment length to accurately localize chromatin features. <a href="https://www.jove.com/files/ftp_upload/52535/52535fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/52535/52535fig3highres.jpg" /> 
Figure 3.&#160;An example of post-sequencing analysis and data visualisation by means of the zygotic T-box transcription factor VegT (zVegT). All read counts shown here are normalised to 10 million uniquely mapped and non-redundant reads. (A) Excerpt of the genome-wide profile of zVegT binding in X. tropicalis gastrula embryos (stage 11 to 12.5 after Nieuwkoop and Faber29). Each peak, a pile-up of extended reads, represents one binding site. These peaks are called by MACS2 with a false discovery rate (FDR) of less than 1%. Each mesp gene shows very proximal and upstream zVegT binding, but only mespa and mespb are expressed by that stage (RNA-Seq data30). (B) DNA occupancy levels of zVegT as determined by ChIP-qPCR at several loci (including a non-bound region 0.5 kb upstream of &#946;-actin) confirm the specific enrichment found by ChIP-Seq. Compare results for mespa with peak called (red bar) in (A). The DNA occupancy level is visualized as a percentage of input for both, the ChIP with the VegT antibody (rabbit polyclonal of IgG isotype) and the ChIP with the antibody control (normal rabbit IgG). Error bars reflect the standard deviation of two biological replicates. (C) Metagene analysis shows preferential zVegT binding (tags binned over 25 bp) to the promoter relative to any other genomic region around and within gene bodies. (D) Heatmap shows k-mean clustered (k=5) DNA occupancy levels (tags binned over 25 bp) of zVegT and Smad2/Smad3 (ChIP-Seq data31) relative to all zVegT-bound regions at gastrula stage. The heatmap is log2 based and centred at 5 tags per bp. (E) De novo motif analysis discovers the canonical T-box transcription factor binding motif in 38% of zVegT-bound regions if the underlying motif score is normalised to a 5% discovery rate in background sequences. The density map shows highest enrichment for the T-box motif in the centre of zVegT binding sites, whereas the canonical Smad2/Smad3 binding motif is hardly enriched. (F) Histogram shows DNA occupancy levels of zVegT, which are calculated for each target gene from all peaks (+/- 200 bp) between 5 kb upstream [-] and 1 kb downstream [+] of corresponding transcription start sites. (G) Top 300 genes with highest DNA occupancy levels within -5 kb and +1 kb are enriched for biological processes of early embryonic development. These GO terms are in line with the putative function of zVegT. The FDR is based on a two-tailed Fisher&#8217;s exact test and corrected for multiple testing. <a href="https://www.jove.com/files/ftp_upload/52535/52535fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Genome-wide Snapshot of Chromatin Regulators and States in Xenopus Embryos by ChIP-Seq at JoVE.com

Genome-wide Snapshot of Chromatin Regulators and States in Xenopus Embryos by ChIP-Seq

The question of how chromatin regulators and chromatin states affect the genome in vivo is key to our understanding of how early cell fate decisions are made in the developing embryo. ChIP-Seq&#8212;the most popular approach to investigate chromatin features at a global level&#8212;is outlined here for Xenopus embryos.

Genome-wide Snapshot of Chromatin Regulators and States in Xenopus Embryos by ChIP-Seq

The recruitment of chromatin regulators and the assignment of chromatin states to specific genomic loci are pivotal to cell fate decisions and tissue and ...

genome-wide-snapshot-chromatin-regulators-states-xenopus-embryos-chip

Research

JoVE Journal

Developmental Biology

12.1K Views.  MRC National Institute for Medical Research. The question of how chromatin regulators and chromatin states affect the genome in vivo is key to our understanding of how early cell fate decisions are made in the developing embryo. ChIP-Seq—the most popular approach to investigate chromatin features at a global level—is outlined here for Xenopus embryos.